In medical procedures, the movement of a surface of an implantable device with respect to tissue is important in reducing damage to both the surface and to the tissue. Damage to tissue as a result of “tissue drag” friction is know to cause inflammation and pain, and may lead to a longer recovery time. High friction between a surface material of an implant and blood may result in clotting and subsequent occlusion of a blood vessel. Friction may also damage the implant material, thus rendering it ineffective or shortening its useful life.
The problem of “tissue drag” has been of concern to the medical profession for some time. For example, it is know to improve the lubricity of a braided polyethylene terephthalate suture by applying a coating to the outer surface of the suture consisting of polymers of polyethylene or polytetrafluoroethylene (PTFE) having a lower coefficient of friction than the surface of the suture. It is also known that sutures coated with dimethylsiloxane-alkylene oxide copolymers have improved handling characteristics. These polymers, however, are not bioabsorbable, and therefore leave a residue in the tissue.
The use of certain bioabsorbable polymers as coatings to improve the tie-down performance of sutures and to also reduce tissue drag is similarly known in this art. These coatings may include copolymers and blends containing monomers of lactide, glycolide, epsilon-caprolactone, trimethylene carbonate, p-dioxanone, ethylene oxide, and propylene oxide.
The reduction of tissue drag using bioabsorbable polymers as coatings on implantable medical devices other than sutures has also known. The coated devices include, for example, screws and suture anchors, having surfaces that drag along and contact both soft and hard tissue during implantation.
Many implantable medical devices, such as hip or knee prostheses, are structured such that, during their life, or during the medical procedure for implantation, there is movement of a surface of the device against another surface of the device. This relative movement, or articulation, of one surface against another, is known as “device drag”. In device drag, friction may damage the material of the surface, thus rendering it ineffective or shortening its useful life.
The issues of device drag in non-bioabsorbable implantable medical devices have been addressed in a variety of ways. For example, it is known to apply a thin layer of a low coefficient of friction coating (ceramic or diamond-like carbon) on one or more contact surfaces. Such a coating reduces friction between the surfaces of a bone fixing device formed from conventional implantable materials such as titanium alloys and solids ceramics.
Also, with regard to implantable orthopedic prostheses having a metallic first component having a first bearing surface, and a second metallic component having a second bearing surface, where the second bearing surface is disposed in opposition to the first bearing surface in a sliding bearing relationship, it is known to provide on at least one of the first and second bearing surfaces a plurality of substantially evenly distributed plateaus interspersed with valleys. The valleys have a depth of about 0.0002 inch to about 0.002 inch below the plateaus to facilitate lubrication of the articulating surfaces by natural body fluids.
Although the issue of device drag has been addressed in non-bioabsorbable implantable medical devices, there is a desire in many applications to move away from non-absorbable implants. A major disadvantage of non-bioabsorbable implantable medical devices is that they remain permanently in the body. It is known that these implants can cause a variety of problems after healing, for example, chronic irritation and inflammation of surrounding body tissue, abrading of tissue during normal motion of the joint, and problems in X-ray imaging in follow-up examinations since the implant may block out the view of the tissue. When complications do arise from non-bioabsorbable implantable medical devices additional surgical procedures may be required to remove problematic devices once the tissue has healed, placing the patient at additional risk.
Bioabsorbable implantable medical devices are naturally degradable by the body, through known mechanisms including bioresorption and biodegradation. Accordingly, contact with surrounding tissue after implantation does not necessitate surgical intervention because the device will be completely absorbed by the body once the tissue has healed. Reducing device drag is particularly advantageous in polymeric bioabsorbable devices where the device is inserted in hard body tissues such as bone using a driver that engages the device. The driver/device connection or engagement location is susceptible to failure if the load resulting from tissue drag exceeds the strength of such connection or engagement location. By reducing tissue drag, the load necessary to insert the device is typically decreased, reducing the risk of failure at the driver/device connection, or failure to other parts of the device as well.
The problems of tissue drag and device drag in implantable medical devices have been of concern to the medical profession for some time. In implantable devices that drag along tissue, both non-bioabsorbable and bioabsorbable coatings have been reported. In non-bioabsorbable implantable medical devices, there have been attempts to reduce device drag using non-bioabsorbable low friction coatings or surface modification. However, device drag in bioabsorbable implantable medical devices, particularly occurring during implantation, or when there is articulation of surfaces, has been given little attention.
Accordingly, there is a need in this art for methods of reducing device drag in bioabsorbable implantable medical devices while maintaining the bioabsorbable nature of the devices.